1. Field of the Invention
The present invention relates to hydrocarbon-adsorbents capable of effectively adsorbing hydrocarbon (HC) in exhaust gases emitted from gasoline engines or the like.
2. Description of Related Art
Zeolite having another name of molecular si eves has pores of which the dimensions are nearly equal to those of molecules, and consequently has been used as adsorbents and catalysts in many reactions. Zeolite contains cations adapted to neutralize negative charges of aluminum oxide, which are readily exchanged with another cations in aqueous solutions thereof, so as to be used as cation-exchangers, too.
Recently, application of zeolite having the above-described properties to catalysts for purifying exhaust gases of automobiles has been investigated. In Japanese Patent application-laid open No. Heisei 3-232533, for example, a catalyst for purification of exhaust gases, which is composed of zeolite carrying noble metals such as platinum or palladium, is disclosed.
Zeolite itself is, however, low in carrying properties of such noble metals, and consequently, the amount of carried noble metals is too small to exhibit sufficient oxiding ability. Since the catalyst is not activated at 300.degree. C. or less, in case the exhaust gas temperature is low, such as at an engine starting time, HC in exhaust gases cannot be sufficiently purified. When the engine is in the cold state where an air-fuel mixture having a higher fuel concentration than that under normal engine operation, is supplied, the amount of hydrocarbon contained in exhaust gases is great. Consequently, it is desired to purify hydrocarbon effectively particularly when the engine is started or in the cold state.
To this effect, recently, zeolite has been used as an HC-adsorbent. In Japanese Patent application-laid open No. Heisei 5-317701, for example, there is a disclosure that zeolite as an HC-adsorbent, which has a predetermined range of SiO.sub.2 /Al.sub.2 O.sub.3 mole-ratio, is used along with an oxidation catalyst, thereby improving the purifying performance of HC when the engine is started in the cold state.
By using the oxidation catalyst and adsorbent together in the low temperature range, HC is adsorbed by the adsorbent temporarily to restrain the emission thereof. When the temperature of the adsorbent rises to a predetermined temperature or more, the adsorbed HC is emitted, and oxidized and purified with the oxidation catalyst existing near the adsorbent. Thus, HC can be purified stably from the lower temperature range to the higher temperature range.
In addition, in Japanese Patent application-laid open No. Heisei 6-154538, there is a disclosure about the method of disposing zeolite downstream of a three-way catalyst in an exhaust gas flow to adsorb HC in exhaust gases in the low temperature range, and emitting adsorbed HC in the elevated temperature range, thereby purifying exhaust gases. With this method, HC which has been emitted when the engine is started in the cold state can be purified effectively with the three-way catalyst.
And, in Japanese Patent application-laid open No. Heisei 7-96178, there is a disclosure that zeolite composed of at least one of ZSM-5, mordenite, Y-type zeolite and X-type zeolite is used as the HC-adsorbent, and that the preferred SiO.sub.2 /Al.sub.2 O.sub.3 mole-ratio of zeolite ranges from 15 to 250.
The HC-adsorbent using zeolite has, however, the problem that the adsorption efficiency greatly depends on the kinds of HC. More specifically, higher HC of which the number of carbon is 4 or more is adsorbed at a relatively higher rate. In contrast, lower HC of which the number of carbon is 3 or less is difficult to be adsorbed. The HC-adsorbent adapted to be secured to an exhaust system of an engine is also required to exhibit durability at elevated temperatures up to about 800.degree. C.